The invention relates to a method for breaking rock to be drilled in rock drilling, in which method the rock to be drilled is subjected to successive stress waves via a tool in such a way that the energy of the stress wave transmitted from the tool to the rock causes the rock to be broken.
In rock drilling or the like, rock is broken by conducting a stress wave to the rock via a tool, such as a drill rod or a drill bit at its end. A stress wave is nowadays typically generated by striking the end of the tool with a percussion piston moving back and forth in a rock drilling machine or percussion device by means of a pressure medium. In rock drilling, both the supply of a stress wave and the rotating of the tool take place simultaneously, but the breaking of the rock material is actually based on the energy of the stress wave transmitted from the tool to the rock.
Typically, about 50 to 80% of the energy content of the stress wave is transmitted to the rock to be broken. The energy transmitted to the rock material causes macro-cracks, breaking of rock material and elastic waves. The energy bound to the elastic waves is lost with regard to the breaking of the rock material. On the other hand, producing macro-cracks is, with regard to breaking, more efficient than crushing of rock material. Due to the macro-cracks, large particles are detached from the rock material, whereas in crushing the rock material is ground completely fine, which requires a large amount of energy. Thus, it would be preferable to generate as large a number of macro-cracks as possible instead of crushing the rock.
Present percussion devices generate stress waves at a low frequency, typically at 20 to 100 Hz, the length of the stress wave being rather short, i.e. about 0.2 to 1.6 m. At the same time, the amplitude and energy content of the stress wave are high. At the highest, the amplitudes are typically 200 to 300 MPa. Because of the amplitude of the stress wave, it has been necessary to design the button bits to be used to withstand a high load level. Therefore, there have to be a large number of rock-breaking buttons in a button bit, and the buttons have to be designed to withstand load peaks. Their shapes are thus disadvantageous with regard to the breaking of rock. Therefore, what is called the penetration resistance of the button bit, expressing the proportion of the force exerted on the rock by the button bit to the penetration of the buttons, is large.
The high energy level combined with the disadvantageous shape of the buttons leads to poor efficiency in breaking and detaching rock. Correspondingly, high stress wave amplitude values result in a short service life of the drilling equipment used, i.e. drill rods and button bits. It would be preferable, in regard of generating macro-cracks, to be able to use what are called aggressively shaped buttons but this is not feasible at the present stress amplitude level. If it were possible to use such buttons, breaking of rock could be made significantly more efficient compared with the present solutions.
In developing present solutions, the focus has generally been in using greater percussions powers and thus using higher stress wave amplitudes than before. Surprisingly, however, it has been noted that the same result can be achieved with the method according to the invention by using, contrary to the present trend, significantly lower stress wave amplitudes than today.